Spinocerebellar ataxia type 1 (SCA1) is a slowly but relentlessly progressive neurodegenerative disease that first impairs balance and fine coordination, then eventually causes death by robbing its victims of the ability to clear their airway. There is currently no effective treatment. Much has been learned about SCA1 pathogenesis since the discovery, nearly two decades ago, that it is caused by an expansion of the polyglutamine (polyQ) tract in the Ataxin-1 (ATXN1) protein. The polyQ expansion alters ATXN1's native protein interactions but also, more generally, renders the protein more stable and less easily cleared from the cell, with the end result being elevated ATXN1 levels. It turns out that protein levels are crucial in SCA1: genetically decreasing mutant ATXN1 levels or reducing levels of proteins that selectively stabilize the interaction can mitigate the disease. In this context it is noteworthy that elevated levels of even wild-type ATXN1 are toxic, and similar observations have since been made for other neurodegenerative disease proteins (e.g., synuclein, tau, and amyloid beta). These considerations have led us to hypothesize that reducing ATXN1 levels could prove to be an accessible, relatively low-risk mode of treatment. In the interest of identifying druggable targets that could safely reduce ATXN1 levels, we developed an integrated forward genetic screening strategy to scan the kinome, in both human cells and fruit flies, for ATXN1-regulating genes and pathways. (We began with the kinome because ATXN1 phosphorylation at S776 is crucial to its toxicity; even a greatly expanded ATXN1 with a S776A mutation will not cause pathology.) Through the screen we found that the Ras/MAPK/MSK1 pathway modulates ATXN1 levels and that MSK1 phosphorylates ATXN1 directly at S776 in vitro. In Aim 1, I will study the role of MSK paralogs in SCA1 pathogenesis by performing genetic studies of Msk1 and Msk2, in the SCA1 mouse model and evaluating the mice with neuropathological and behavioral assays. In the hope of eventually developing combination drug therapy, which might be more effective and less toxic, I will seek additional ATXN1-modulating pathways in Aim 2. We have searched other parts of the genome beyond the kinome with another screen and I am following up on the primary data generated by performing a secondary screen to validate and prioritize gene candidates (Aim 2A). For the two most promising candidates, I will test their roles in Atxn1 regulation in SCA1 mouse brain using adeno-associated virus (AAV)-mediated gene silencing, a technology that I have developed and optimized for screening Atxn1-regulators in vivo (Aim 2B). The findings from these proposed experiments will not only deepen our understanding of SCA1, but should lead to rational treatments, as many kinases and candidates from the screen are already druggable targets. Discovering pathways that modulate ATXN1 and demonstrating the success of the screening strategy in this disorder will also establish the usefulness of this approach for other more common neurodegenerative diseases caused by increased levels of the causative proteins, such as Huntington's, Alzheimer's, and Parkinson's disease.